The present invention relates to a time-division multiplex pulse-code-modulation (PCM) system for use in combining multiple lower-order digital data pulse streams into a single higher-order data pulse stream, and more particularly to a multiplexer apparatus for multiplexing multiple digital data pulse streams whose frames are aligned in phase with each other.
A multiplexer apparatus of this kind includes synchronizers for each channel, which receive data pulse streams having the identical bit rates and aligns the data pulse streams in frame with each other to produce frame-phase synchronized data pulse streams; and a multiplexing unit for multiplexing them into a multiplex data pulse stream. Accordingly, an increased number of channels involves an increased number of synchronizers and makes it difficult to locate each synchronizer under the same electrical conditions with respect to the multiplexing unit. Specifically, when the synchronizers have to be laid out apart from the multiplexing unit in a distributed manner in a housing frame, there are differences in lengths of cables that interconnect each synchronizer with the multiplexing unit. These differences cause differences in time delay. Therefore, such an arrangement involves losses in bit synchronization and frame-phase synchronization among once frame-phase synchronized data streams prior to their arrival to the multiplexing unit. The losses in such synchronizations make the multiplexing unit unable to operate properly manner. These problems become more serious with increased data pulse rate.